The ragged barrier after entering the concurrent mark phase is
vestigial. This used to be the point where we enabled write barriers,
so it was necessary to synchronize all Ps to ensure write barriers
were enabled before any marking occurred. However, we've long since
switched to enabling write barriers during the concurrent scan phase,
so the start-the-world at the beginning of the concurrent scan phase
ensures that all Ps have enabled the write barrier.

Hence, we can eliminate the old "install write barrier" phase.

Fixes #11971.

Change-Id: I8cdcb84b5525cef19927d51ea11ba0a4db991ea8
Reviewed-on: https://go-review.googlesource.com/16044Reviewed-by: Rick Hudson <rlh@golang.org>